Thyrotropin-releasing hormone causes loss of cellular calcium without calcium uptake by rat pituitary cells in culture. Studies using arsenazo III for direct measurement of calcium.
Thyrotropin-releasing hormone (TRH) may act to stimulate prolactin secretion by increasing the intracellular free Ca2+ concentration. This notion is supported by the finding that TRH acutely enhances 45Ca2+ efflux from pituitary cells which may reflect alterations in Ca2+ influx or efflux, or both. To differentiate among these possibilities, we measured loss and uptake of nonradioactive Ca2+ by GH3 cells, a cloned strain of rat pituitary cells that produce prolactin, during TRH action using the metallochromic indicator arsenazo III. Cells were perfused in medium containing 2.8 microM Ca2+ and nonradioactive Ca2+ was measured in the perfusion effluent. Under these conditions, there was a sustained loss of Ca2+ from the cells for at least 30 min. TRH caused a transient, marked increase in the amount of Ca2+ released into the medium which occurred in parallel with enhancement in 45Ca2+ efflux and stimulation of prolactin secretion. There was no measurable decrease in Ca2+ concentration in the medium at the onset of the TRH effect which would have been consistent with Ca2+ influx into the cells. Furthermore, an identical response to TRH was observed in cells perfused with medium containing 50 microM verapamil, an agent which blocks Ca2+ influx. In static incubations performed in parallel, TRH caused a decrease in total cellular Ca2+ of 23 +/- 5%. These data provide direct evidence that TRH causes loss of Ca2+ from GH3 cells without causing measurable Ca2+ uptake and support the contention that TRH acts by mobilizing Ca2+ from a sequestered cellular pool (or pools).
D000818:Animals; D001150:Arsenazo III; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D007700:Kinetics; D009374:Neoplasms, Experimental / Q000378:metabolism; D010911:Pituitary Neoplasms / Q000378:metabolism*; D051381:Rats; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
D001150:Arsenazo III; D013973:Thyrotropin-Releasing Hormone; D002118:Calcium